An electrostatic precipitator generally comprises a housing structure having a gas inlet and a gas outlet and, between the gas inlet and gas outlet, a stack of mutually parallel but spaced-apart collector electrodes which are generally planar and disposed with a vertical orientation. Beneath this stack there may be provided a bin into which the collected particles pass when the electrodes are jolted by a rapping device. Between the dust-collecting electrodes, corona discharge wires may be provided in parallel arrays to charge the dust particles so as to enable them to be attracted to the collecting electrodes.
In recent years, as the demand for electrostatic precipitators has developed with increasing concern for environmental pollution, particular collector electrode arrangements have gained prominence.
In the commonly assigned U.S. Pat. No. 3,803,809 issued Apr. 16, 1974, for example, there is described by one of us, among others, an advanced construction of the collecting electrode assembly in which each of the collecting electrode walls consist of a multiplicity of individual metal strips, the contiguous portions of adjacent strips having loosely interengaging edges. The strips forming each of the plates are suspended from their tops from a common carrier or beam and are secured at their bottoms to a common bar upon which acts the rapping linkage.
The outer edges of each plate, i.e. the free edges of the two strips lying along the vertical edges of the plate, are held against rotation by claw-like profiled members which embrace these edges. As disclosed in the aforementioned patent, these profile edge members prevent rotation of the electrode walls and extend substantially the entire height of the strips which they engage.
It was pointed out in said patent, moreover, that difficulties arise in the removal of dust from exhaust gases when pyrophoric fine dust collectors on the electrode and flameless combustion takes place in local areas of the collecting electrodes.
These localized excess heating tends to relax internal stresses which are present in the collecting-electrode strips as a result of their formation by cold-rolling. The electrode strips, which are narrow and high, thus tend to be distorted and twisted so that the width of the discharge gap between them and the array of corona-discharge or ionizing electrodes is decreased and increased at various locations. Any decrease in the gap means that a lower operating voltage must be used especially where the operating voltage is close to the breakdown voltage which would result in a continuous arc between the collecting electrode strip and the ionizing electrode. As the gap width decreases, therefore, the effective operating voltage and certainly the maximum possible operaint voltage decreases.
As the voltage level is reduced, there is a corresponding reduction in the effectiveness of particle removal from the gas and the degree from which particles are removed.
It is for this reason that the system of the aforedescribed patent provides distortion-limiting channels over the entire height of the electrode strips at the edges of each plate. These twist-resisting channels are forceably moved into positions in which they embrace the exposed outer edges of the collecting electrodes, thereby retaining them against rotation.
As a consequence, the electrodes cannot be distorted to any significant degree and cannot twist to a detrimental extent.
With increases in the size of industrial operation, e.g. larger cement mills, sintering plants, power plants and the like, there has been a corresponding need for larger dust-collecting electrostatic precipitators as noted above.
The most effective way of increasing the size of an electrostatic precipitator for a given area to be occupied is by increasing its height so that the electric field and hence collecting electrode plate heights of 15 meters and more are not uncommon. When high gas velocities are employed with such plates, even with the techniques described in the aforementioned patent, problems have been found to arise.
For example, at gas velocities in excess of two meters per second, vibration is excited in the long electrode strips so that the distance between the collecting electrodes and the array of corona-discharge electrodes is periodically or aperiodically varied with the same detrimental effect as if the strips were distorted. This phenomenon is analogous, therefore, to the phenomenon resulting from flameless combustion and equally limits the maximum operating voltage and hence the maximum degree of particle separation from the gas.